The present invention relates to a method and apparatus for the restoration of a tooth with a fractured cusp or a tooth that has been previously restored on several surfaces.
In such situations, the tooth is usually weakened and in need of crown restoration because, after the tooth has been excavated by the removal of old restoration material and/or decayed or defective tooth structure, there is often insufficient structure left to allow the utilization of a conventional cast crown. To address this problem, modern dental practice generally utilizes one of three techniques for restoring a tooth which is in a weakened condition as described above. These three techniques are known as:
(a) cast crown with parallel iridio-platinum or cast pin retention utilizing cemented pins;
(b) combination of a pin-retaining device, which may be a cemented, friction lock or self-threading pin, and amalgam buildup restoration;
(c) pin-retained foundation of amalgam or composite resin covered with a cast crown.
Each of these techniques has its disadvantages, which disadvantages do not exist with the method and apparatus taught herein. In particular, with respect to technique (a), cemented pins are substantially less retentive than self-threading pins or friction lock pins. This technique requires much impression-taking and model-construction utilizing the pins. Further problems include seating the crown accurately, drying and varnishing the pin holes in the dentin for the pins, and cementing the crown to the pins.
With respect to method (b), there are two main disadvantages namely, (i) the difficulty of rebuilding the tooth into proper occlusion, while retaining good natural contacts and contours; and (ii) the inherent weakness of amalgam to survive for long periods of time in the mouth. It is believed that the compressive strength of this type of restoration is not increased by the placement of pins. Additionally, the pins used for this technique decrease the transverse and tensile strength of amalgam. This technique also requires the placement of multiple pins. It is known that when more than one pin is placed in a tooth, the potential for dental crazing, damage, fracture, pulp devitalization and tooth loss increases substantially.
The main disadvantage of technique (c) is the hydrophilic property of all commercially available composite resins which which results in the resin material absorbing water and oral fluids. Any space between the restored tooth and the composite resin becomes filled with fluid when contacted by water or saliva. Once wet, it is difficult, if not impossible, to dry the composite resin in the space before cementation. The moisture causes the cement to be leached out and leaves it chalky and permeable. Hydrophobic fluorindated composites have been developed, but are not presently commercially available.
The present invention avoids most, if not all of the disadvantages of the prior art techniques and, as will be discussed below, has many advantages over the prior art techniques.
Prior art references of interest are as follows:
U.S. Pat. Nos. 400,921, 1,018,803, 3,675,328, 3,728,794, 3,831,281, 4,202,101, 4,259,076 and 4,331,423 each of which teaches pin or screw anchoring devices used for dental restorations.